I don’t know the original source, but it popped into my head yesterday:
“A chicken is an egg’s way of making more eggs.”
It touches on another important metaphor that’s useful for understanding evolution: The selfish gene. I haven’t read the Dawkins book myself, but I think I’ve got the general idea down. Evolution is more about genes than organisms, which is part of why altruism shows up in nature. Kin selection is where animals seek to aid members of their families. Your siblings likely share many of the same genes you do, so if there’s a gene or combination of genes that causes its organism to have protective instincts toward its siblings, the gene sequence is “selfishly” protecting copies of itself, even though they’re in an entirely different organism. Altruists can form trust among each other and join into groups, which gives them abilities they wouldn’t have as individuals. They can end up becoming dependent on being in a group, which means the altruism will be a conserved trait, since losing it means losing the group they’re dependent on. Groups can tolerate some selfish behaviors, but there’s going to be a limit of some kind enforced, whether it’s through punishments for selfish individuals or social breakdown.
The fun part is that, with our large brains, humans are capable of taking altruism and running with it. We can consciously appreciate the benefits detached from genetics, ironically rebelling against the evolutionary “purpose” in being altruists. We’ve ended up as memetic specialists: A huge chunk of our survival ability is based on our cultural heritage and modification, not just our genes. We value people for their individual character, not their reproductive potential. We can appreciate people for the ideas they can pass on. We can appreciate the value of alternative perspectives, including those that come from the disadvantaged. We can afford to be generous toward so-called “inferior” people because culture is such a huge benefit to our species, and you don’t need to be disability-free to contribute to that culture.